The present invention relates to a focus error detector which is mainly adopted for an optical pickup for recording and reproducing information by scanning a recording medium and which detects a signal representing the degree of a focus error of an objective lens with respect to the recording medium.
In an optical recording and reproducing pickup, the optical structure of an optical pickup adopting an improved focus error detector in the prior art is shown in FIG. 1.
The light radiated from a laser diode 1 used as a light source is collimated by means of a collimating lens 2 and then passes through a beam splitter 3 to then be incident to an objective lens 4. Thereafter, the light is focused onto an optical disk 5 by objective lens 4. After the reflected light 6 from optical disk 5 is collimated again via objective lens 4, the light travels toward beam splitter 3 to then be focused by a focusing lens 7. The reflected light focused by focusing lens 7 is split into first and second reflected light rays 6a and 6b by means of another beam splitting member 8. The first and second reflected light rays 6a and 6b are focused onto focal points Fa and Fb of focusing lens 7, respectively, and are received by two bi-segmented photo-detectors 9 and 10 each arranged in front and rear portions of the focal points Fa and Fb. The two bi-segmented photo-detectors 9 and 10 are positioned so that their dividing boundaries are slightly askew from each optical axis. Meanwhile, two differential amplifiers 11 and 12 and an summing amplifier 13 perform an operation with respect to the signals from two bi-segmented photo-detectors 9 and 10 to then supply a signal indicating the degree of the focus error corresponding to a relative distance of objective lens 4 with respect to optical disk 5.
In the aforementioned conventional focus error detector, when optical disk 5 is positioned on a focal plane of objective lens 4, the reflected light 6 incident to focusing lens 7, which is collimated, is focused to each given position of focal points Fa and Fb of focusing lens 7. Therefore, at this time, as shown in FIG. 3A, spots having the same size are formed on two bi-segmented photo-detectors 9 and 10. Here, since the photo-detectors 9 and 10 are deviated from each optical axis, differential amplifiers 11 and 12 for differentiating both split sections outputs signals having opposite polarities and but having the same absolute value. Therefore, the output of summing amplifier 13 is zero. When the aforementioned optical disk B is deviated from the focal plane of objective lens 4, the reflected light 6 incident to focusing lens 7 is diverged or converged to then be focused somewhere other than at focal points Fa and Fb of focusing lens 7. Then, the spots of first and second reflected light rays 6a and 6b respectively formed on photo-detectors 9 and 10 become relatively larger or smaller with respect to each other, as shown in FIGS. 2B and 2C. Therefore, at that time, the finally output focus error signal has a negative (-) or positive (+) value.
On the other hand, in the conventional focus error detector, if the optical axis of the reflected light is tilted or shifted by the optical disk vibration or changes over time, even if the optical disk is positioned on the focal plane of objective lens, the spots on the respective photo-detectors are shifted to generate a deviation in the light amount received by the split sections. Due to such a deviation of the light amount, the final focus error signal is output as a negative (-) or positive (+) value, that is, not zero. That is to say, the conventional focus error detector undergoes frequent malfunctions by the optical axis tilt due to the optical disk vibration or changes over time. Also, since the angle formed by the two reflected light rays split by the aforementioned beam splitter is 90.degree. and one of the two photo-detectors for receiving the two reflected light rays are positioned outside the focal points of focusing lens, much space is occupied, which make miniaturization difficult.